Nine new isolates of Giardia have been axenized and are in the process of being studied. The DNA of other isolates were analyzed by endonuclease restriction analysis and found to be different from most of the previously defined Giardia isolates. In order to study the structure and gene organization of the major 170kD antigen of isolate WB, the gene was cloned into an expression vector and a number of clones isolated. Sequencing of this gene is in progress. The insert from this vector was cloned into M13 and used as a probe in restriction endonuclease studies of various isolates. The gene is present in isolates that do not express the 170kD antigen. Mutants of Giardia could not be produced by commonly used procedures. Transfection of Giardia was attempted with Icarus hs neo gene which confers resistance to G418. Although RNA transcripts were found, no detectable translated products were found. The heat sock gene (hs-p) of Giardia was isolated and cloned into Puc18 and the promotor for the gene will be used with the neo gene. Translation products of RNAs from different isolates showed subtle but definite differences. A 43kD product was recognized by infected human and gerbil sera.